Method and apparatus for quantum measurement via mode matched photon conversion

ABSTRACT

The present disclosure relates to a generally-applicable measurement technique based on coherent quantum enhancement effects and provides embodiments with nonlinear optics. The technique utilizes parametric nonlinear processes where the information-carrying electromagnetic quanta in a number of electromagnetic modes are converted phase coherently to signature quanta in a single mode or a few modes. The phase coherence means that while the quanta before conversion may have unequal or uncertain phase values across the modes, the signature quanta converted from those different modes have the (near) uniform phase. This can lead to significant increase in the signal to noise ratio in detecting weak signal buried in strong background noise. Applications can be found in remote sensing, ranging, biological imaging, field imaging, target detection and identification, covert communications, and other fields that can benefit from improved signal to noise ratios by using the phase coherent effect.

CROSS-REFERENCE TO RELATED APPLICATION

This application relates to U.S. Provisional Patent Application No.62/427,604 filed Nov. 29, 2016, the entire disclosure of which isincorporated herein by reference.

FIELD OF THE INVENTION

The present disclosure relates to method and apparatus adapted forquantum metrology, remote sensing, imaging and covert communications.

BACKGROUND OF THE INVENTION

Measurement devices based on the detection of reflected lightwave,microwave, or radio signals can take dynamic, 3D images of remotetargets, giving rise to a wealth of robust applications, such as remotesensing, ranging, biological scanning, field imaging, stand-off targetidentification, and vehicle speed determination. In a typicalconfiguration using radio-wave signals-widely known as radio detectionand ranging, or radar-a radio wave beam is created and emitted along adirection of interest. By measuring the reflected radio wave, thepresence of an object over distance can be inferred, maybe along withinformation on the object's shape, speed, and so on.

Light detection and ranging, or lidar, operates on a similar principlebut uses light waves in place of radio waves. As light waves have muchshorter wavelengths than radio waves, lidar can generally offersignificantly better coordinate precision and imaging resolution.

Radar and lidar are two exemplary, but not the only measurementtechniques based on the detection of reflected signals. In suchtechniques, however, a major limitation on performance arises from thepresence of background noises. When background noises share the samespectral band and arrival time with the reflected signals, it isdifficult, if not impossible, to distinguish them from theinformation-carrying signals, resulting in false alarms, incorrectobject identification, distorted images, and so on.

For many measurement devices, the performance is ultimately limited bythe signal to noise ratio (SNR) as detected. In addition, when classicalbeams (e.g., a laser beam, a radio wave) are used, an adverse party canmeasure the emitted signal and generate a mimicking signal and send itback to cheat the detection device. This method of attack is sometimesreferred to as “spoofing.” While techniques like pulse shaping andmodulating can make the spoofing difficult to implement in practice,they do not exclude or detect the presence of spoofing.

To improve the SNR, “quantum illumination” (Q) has been proposed, inwhich a pair of entangled photons are prepared in M (M is much greaterthan 1) electromagnetic modes. One photon in the pair is sent out fordetection, and upon reflection, the returning photon is jointly detectedwith the other photon kept in place. By a pure mathematical argument, itwas shown that the SNR can be increased by a factor of M. This idea wasfollowed with expanded analyses proposing a specific realization usingGaussian states.

In various techniques based on QI or its derivatives, entangled quantumstates in photons are used. However, quantum entanglement in photons issusceptible to loss, scattering, and/or environmental disturbances.Thus, while a significant advantage in SNR can be established perreturning photon over the classical systems (e.g., those of lidar andradar) using the same total amount of photons for detection, theperformance of those quantum systems in practice is often worse than theclassical systems, as the latter can instead deploy bright beams of muchmore photons for detection. To understand this, consider using a singlepair of photons in M modes (assuming M ^(˜)1000 under typical settings)in a typical QI application. Per reflected and detected photon (whichoccurs at a very low probability in typical environment settings due toscattering loss, absorption, and so on), the quantum-entanglement basedtechniques can in theory achieve about M-times better SNR than aclassical system. However, the classical system can use many photons,say N photons, simultaneously per detection trial, instead of just asingle photon in the QI application, so that the probability ofsuccessfully obtaining a returned signal (i.e., a reflected photon) is Ntimes stronger. For a typical bright optical pulse in the visiblespectral band with 1 watt peak power and 1 ns pulse duration, there areN^(˜)10 billion photons contained in the pulse, which is much largerthan M. For the same background noise level, the detected SNR cantherefore be higher for the classical systems than the quantumentanglement-based systems, such as QI and its derivatives, because muchmore photons can be sent out for detection per detection trial for theclassical systems. This deficiency restricts the utilities ofquantum-entanglement based measurement systems.

SUMMARY OF THE INVENTION

In view of the foregoing background, a measurement technique based oncoherent quantum-mechanical enhancement effects is disclosed. Thetechnique utilizes parametric nonlinear processes whereinformation-carrying electromagnetic quanta (e.g., optical photons,microwave photons, radio photons) in a large number of electromagneticmodes are converted phase coherently to signature quanta in a singlemode or a few modes. The phase coherence means that while the quantabefore conversion may have unequal or uncertain phase values across themodes (such as the quanta in different modes have nonzero, stochasticrelative phases between each other), the signature quanta converted fromthose different modes to the same mode have a (near) uniform phase,i.e., the relative phase values for most quanta are close to zero, wellwithin 180 degree. This phase coherence can lead to significant increasein the signal to noise ratio in detecting weak signals buried in strongbackground noises.

In one embodiment, a beam of signal quanta in M modes (M being greaterthan one), each paired with a quantum in a separate idler beam, iscreated in a nonlinear device to use as a probe for measurement. Theaverage number of quanta in each signal or idler mode can be less,equal, or greater than one. The relative phases between signal quanta indifferent modes are random. The relative phases between idler quanta indifferent modes are random, too. However, each signal quantum is createdwith an idler quantum to form a pair, where the sum or differencebetween the phases of the paired signal and idler quanta is uniformacross different modes. The signal beam is sent to interact with aremote target of interest while the idler beam is retained in place. Thereflected signal quanta, which are randomly distributed in differentmodes, are combined and temporally aligned with the idler beam. Here thetemporal alignment means that the mode of each signal quantum overlapswith the mode of its paired idler quantum, similarly to how the twooverlap when they are created in pairs in the first place. The combinedbeams are then passed through a nonlinear device, during which thesignal quanta are converted by the idler beam to create signature quantain a single mode through a parametric nonlinear process. When thegeneration and conversion of the signal are carried out in duringnonlinear processes of identical or similar nonlinear properties, theconversion is phase coherent. The resulting conversion efficiency ishigher for the reflected signal than the accompanying background noises,even when the noises share the same modes with the reflected signal,including with the same arrival time, over the same spectral band, inthe same polarization, and with the same orbital angular momenta. Thecreated signature quanta are subject to mode-selective detection toensure that only those in the desired mode(s) are registered in themeasurement. Based on the measurement results, the information of thedetected object(s), such as the distance from the detection device, thesurface optical properties, and the speed, can be inferred.

The disclosed techniques can substantially improve the per-photon SNRsimilarly to how the QI and its derivative techniques can outperformtheir classical counterparts. However, unlike those based on QI or useof few-photon entangled states, the disclosed techniques can use insteadbright twin beams, each containing many photons per measurement trial,such as those consisting of nanosecond pulses with one-watt peak powerand 100 kHz pulse repetition rate. The twin beams do not need to beentangled, which contrasts strongly with those techniques and systemsrelying on having quantum entanglement in the photons. Thus, thedisclosed techniques can lead to system performance substantiallyimproved from that of the classical systems (e.g., radar and lidar)because of the higher SNR per reflected and detected signal photon, andsubstantially improved from those quantum-entanglement based systemsbecause much more photons can be used per pulse. In one embodiment, thenumber of photons per pulse can vary arbitrarily from about 10 to overabout 1 trillion, depending on the needs of specific applications.

The disclosed embodiments have many potential applications, including,but not limited to, the following: radar and lidar under low-visibilityconditions where signal loss is strong; detection of remote objects withlow reflectivity of radar/lidar signals, including stealthy aircrafts,semi-transparent objects, and other targets with reduced-reflectioncoatings; radar and lidar over extended distances; covertcommunications; imaging of a remote object in presence of strongatmospheric noises and disturbances; target surface profiling overextended distances; deep tissue imaging, where 3D images of deep tissuecan be reconstructed through timing the reflected signals, even in thepresence of strong loss and scattering; and self-driving automobiles,where accurate 3D images of the surrounding environment must be obtainedreliably despite unpredictable background disturbances, includingforeign light illumination on the detector whose strength is much higherthan the reflected signal.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, reference ismade to the following detailed description of embodiments considered inconjunction with the accompanying drawings, in which:

FIG. 1 is a schematic diagram of a quantum measurement process performedin accordance with an embodiment of the present invention, the processusing a nonlinear optic setup;

FIG. 2 is a schematic diagram of a quantum measurement process performedin accordance with another embodiment of the present invention, theprocess using a nonlinear optics setup with an idler beam amplified andreturning signal modes corrected for mode distortion;

FIG. 3 is a diagram illustrating parametric downconversion andupconversion in a single waveguide;

FIG. 4 is a schematic diagram of a quantum measurement process performedin accordance with another embodiment of the present invention, theprocess using nonlinear optic setup and a signal beam created by usingpumping pulses and a seedling idler beam;

FIG. 5 is a schematic diagram of a covert communication processperformed in accordance with an embodiment of the present invention,where the signal is buried in a strong broadband noise beam; and

FIG. 6 is a schematic diagram of a covert communication processperformed in accordance with an embodiment of the present invention,where the signal is hidden in the ambient light.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The following disclosure is presented to provide an illustration of thegeneral principles of the present invention and is not meant to limit,in any way, the inventive concepts discussed herein. Moreover, theparticular features described in this section can be used in combinationwith the other described features in each of the multitude of possiblepermutations and combinations contained herein.

All terms defined herein should be afforded their broadest possibleinterpretation, including any implied meanings as dictated by a readingof the specification as well as any words that a person having skill inthe art and/or a dictionary, treatise, or similar authority would assignthereto.

Further, it should be noted that, as recited herein, the singular forms‘a,’ “an,” and “the” include the plural referents unless otherwisestated. Additionally, the terms “comprises” and “comprising” when usedherein specify that certain features are present in that embodiment;however, these terms should not be interpreted to preclude the presenceor additional of additional steps, operations, features, components,and/or groups thereof.

In one embodiment of the present invention, a nonlinear device is pumpedby optical pulses in a single electromagnetic mode to create twin beams,signal and idler, through a nonlinear parametric process, eachcontaining a large number of photons spreading over many electromagneticmodes. The average photon number per mode can be larger than, equal to,or smaller than one. Because of the phase matching condition requiredfor the twin-beam generation, each photon in the signal beam is pairedwith a photon in the idler beam. Here the phase matching conditionrefers to the conservation of energy and momentum during the nonlinearparametric process for the twin-beam generation. In the Hilbert spacewhose state bases correspond to the normal modes of the nonlinearparametric process (which are also referred to Schmidt modes obtainedfrom the Schmidt decomposition of the output twin-beam quantum states),the photons are in paired time-frequency, polarization, and/or spatialdegrees of freedom. In this realization, the twin-beam generation canstart from a spontaneous emission process, such as spontaneousparametric downconversion, followed by parametric amplification of thetwin beams. Alternatively, the twin-beam generation can start with aseedling signal or idler beam as the input to the nonlinear parametricdevice. The seedling beam needs to contain photons in many modes. Insome embodiments, the seedling beam can be created through amplifiedspontaneous emission in a separate device. The above are just twoexamples to create the desired twin beams. There are alternate ways. Forexample, the signal beam and idler beam can be produced and manipulatedseparately such that each pair mode are phase coherent with other pairmodes, similarly to the phase coherence resulting from their creationthrough the nonlinear parametric process driving by a single-mode pump.

The signal beam is then used as a probe and sent out for detection.Meanwhile, the idler beam is stored in a delay line or a buffer/memorydevice. The storage time is chosen to match the round-trip time for thesignal to go out and return after reflection by the target. When thetarget's location is unknown, the storage time is scanned to look forthe returning signals. The basic requirement for the storage is that theno significant amount of noises, such as phase noises, the backgroundphotons, and the uncontrolled distortion of the mode profiles, are addedto the output idler beam.

Depending on the need, the idler beam can be amplified, such as using anoptical parametric amplifier, a semiconductor optical amplifier, a Ramanamplifier, an Erbium-Doped Fiber Amplifier, and a Praseodymium-DopedFiber Amplifier. In one embodiment, the amplification does not introduceany significant amount of noises, such as phase noises, the backgroundphotons, and the uncontrolled distortion of the mode profiles, to theamplified idler beam.

The returning (reflected) signal and idler beams are relatively delayedto overlap in time (e.g., by temporal alignment) as they are co-producedin the first place. The temporally aligned beams are combined in anonlinear parametric device, whose settings is such that paired photons(in different modes) from each beam are combined to phase coherentlycreate new, signature photons in the same mode (e.g., with a shiftedcarrier frequency). The nonlinear device can be the same device forcreating the twin beams, or a separate device but yielding similar phasematching properties.

In some embodiments, the returning signal beam could experiencesignificant mode distortion, such as that caused by group velocitydispersion, heat scintillation, and multi-scattering in atmosphere. Insuch embodiments, the signal beam can first be guided through a linearoptical system, such as a piece of dispersion-compensation fiber, tocompensate for the mode distortion, before they are combined with theidler beam.

The created signature photons are then passed through an electromagneticfilter or a filter set, which selects only those photons in theelectromagnetic modes identical or very similar to that of the pumpcreating the twin beams. Only those photons surviving the filtering willthen be detected by, for example, photodiodes, photomultiplier tube, orsingle-photon detectors.

The detection results are then recorded and analyzed. By analyzing theround-trip time of the reflected signal, the distance of the detectedobject can be inferred. By scanning the launching and receiving angle ofthe signal beam, the transverse coordinate of the detected object(s) canbe inferred and the surface profile of the object(s) can bereconstructed. By analyzing the strength of the returning signal,information about the surface/material properties of the detected objectmay also be obtained.

The above technique is generally applicable to the systems of optics,microwaves, radio waves, or a hybrid mixture of them. For example, thesignal and idler can both be in the optical spectra, or both in theradio spectra, or one in the optical and the other in radio spectralregions.

When the signal and idler beams span over many electromagnetic modes, asignificant advantage in the measurement SNR can be established overclassical methods, e.g., those without using the signal-idler twinbeams. In one embodiment, in which the photons in paired signal andidler beams are equally distributed in M modes, the SNR may be improvedby a factor of about M. On a per-photon basis, this improvement factoris similar to the QI technique and its variants, which are based on theuse of single pairs of entangled photons. The distinct advantage of thedisclosed technique comes from the fact that each mode of the twin beamscan contain many photons while still maintaining the improvement on aper-photon basis. Thus, compared with those based on entangled photons,a much brighter probe beam, i.e., a beam with a large amount of photonsper pulse, can be used so that more signal photons can be reflected anddetected, thus giving rise to substantially enhanced performance.

The procedural steps discussed above relate to an implementation of thetechnique in accordance with embodiments of the present invention. Thereare other forms of implementations. In various exemplary embodiments,implementations may be based on the following principle: whenelectromagnetic quanta in different electromagnetic modes are convertedto create new quanta in the same mode, the generation of the new quantawill be enhanced if the conversion is a phase coherent process. Here,the phase coherence refers to the new quanta converted from differentmodes having the same phase, i.e., the relative phase being zero ornearly zero. By contrast, if the conversion from different modes isphase incoherent, the generation of new quanta would be less efficient.

By way of example, when the idler beam is temporally aligned andcombined with the returning signal beam in the nonlinear device ofidentical or similar nonlinear properties with the device creating thesetwin beams, the signal photons in different modes can be converted tosignature photons in a single mode or a few modes that resemble those ofthe pump that creates the signal and idler beams in the first place. Inother words, the modes of the signature photons will be identical orsimilar to the pump modes in terms of carrier frequency, temporalprofile, spectral profile, and so on. This conversion process is phasecoherent across the many modes the signal photons can be in, as it isthe time-reversal of the parametric twin-beam generation process throughwhich the signal photons are initially created along with the idlerphotons. Hence, the conversion efficiency is high. By contrast, applyingthe same conversion process to the background photons, the conversionefficiency will be much lower due to the absence of phase coherence,even when the background photons share the same spectral band andarrival time with the true signal photons. Thus, the improvement in SNRin the disclosed technique relies on (i) generating a pair of signal andidler beams in well paired electromagnetic modes; (ii) phase coherentconversion of the reflected signal photons in multiple modes tosignature photons in a single mode or a few modes by utilizing the idlerbeam; and (iii) discriminative detection of the signature photons in thesingle mode or a few modes.

There are multiple approaches to satisfying conditions (i) and (ii). Oneembodiment involves utilizing a parametric nonlinear device where brighttwin beams are created by a train of pumping pulses, each pulsecorresponding to a single electromagnetic mode. The twin beams arecreated by spontaneous photon-pair emission followed by parametricamplification. Another embodiment involves utilizing a parametricnonlinear device, a train of pumping pulses in a single mode, and aseedling signal or idler beam with photons in many electromagneticmodes. Passing through the nonlinear device, the seedling is amplifiedand a conjugate beam is created. In these embodiments, the nonlineardevice has broadband phase matching whose bandwidth is many times largerthan the spectral width of the driving pump pulses. Exemplary deviceshaving this capability include lithium-niobate waveguides and crystals,potassium titanyl phosphate waveguides and crystals, lithium triboratewaveguides and crystals, barium borate waveguides and crystals, opticalfibers, silicon nanowires, and amorphous silicon nanowaveguides.

For the phase coherent conversion, the idler beam is aligned andcombined with the reflected signal in the same or a similar nonlinearparametric device with similar nonlinear optical properties to createthe signature photons.

Condition (iii) can be met by applying time and bandwidth limitedfilters to the signature photons before detection, such as a timeshutter followed by a spectral filter. An example is shown in Y.-P.Huang, J. B. Altepeter, and P. Kumar, “Heralding single photons withoutspectral factorability,” Phys. Rev. A 82, 043826 (2010), the disclosureof which is incorporated by reference herein in its entirety. It canalso be realized by using quantum frequency conversion, an example ofwhich is shown in Y.-P. Huang and P. Kumar “Mode-resolved PhotonCounting via Cascaded Quantum Frequency Conversion,” Optics Letters 38,468 (2013), the disclosure of which is incorporated by reference hereinin its entirety. Another example showing the realization by usingquantum frequency conversion can be found in A. Shahverdi, Y. M. Sua, L.Tumeh, and Y.-P. Huang, “Quantum Parametric Mode Sorting: Beating theTime-Frequency Filtering,” Scientific Report 7 6495 (2017), thedisclosure of which is incorporated by reference herein in its entirety.

In one embodiment, all of conditions (i)-(iii) discussed above aresatisfied simultaneously in order to improve the advantage in SNR.

FIG. 1 shows an exemplary implementation of the technique in the opticaldomain in accordance with an embodiment of the present invention. Anoptical pump pulse train in a single spatial mode is generated by apulse generator 10 and is passed through a single-mode filter 12, whichensures that each pump pulse is in only a single time-frequency mode.The pump pluses are then used to create correlated beams (i.e., signaland idler beams) in an optical parametric downconverter 14. The signalbeam is then sent to interact with a target, while the idler beam isretained in a delay line 16, which, in one embodiment, is made of a setof optical fiber loops capable of switching in and switching out opticalpulses. By controlling the total time the idler beam is kept inside thedelay line 16, the idler beam and the returning signal beam can betemporally aligned and combined in a parametric upconverter 18, in whichtwo photons, each in a paired signal and idler mode, can be combined tocreate a new signal photon at sum frequency of the two photons. Thesignal photons are then passed through a single mode filter 20 and aredetected a detector 22.

FIG. 2 shows another exemplary implementation involving proceduressimilar to those illustrated in FIG. 1, with the following differences.First, after its production, the idler beam is passed through an opticalmemory device 16 a followed by an optical amplification device 16 b. Theoptical memory device 16 a is to delay the idler beam (i.e., idlerpulses) so that they can be temporally aligned with the returning signalbeam (i.e., signal pulses). The optical amplification device 16 b isadapted to amplify the idler pulses so that their optical power issufficient to effectively convert the returning signal pulses. For theconversion to be efficient, the output idler pulses need not toexperience significant mode distortion, including any significant changein their temporal pulse shapes or spectral profiles, and any significantincrease in the background noise. If there is significant modedistortion, the idler beam may undergo additional optical devices toadequately compensate for such distortion. Second, in some applications,especially those involving transmission of the signal beam over a longdistance in atmosphere, the returning signal pulses may be in distortedmodes, meaning that their mode profiles of spatial, temporal, orspectral modes may be significantly different from their modes asinitially created in the parametric downconverter 14. In such a case,before being combined with the idler beam, the signal beam may passthrough one or more mode-restoration devices 24 to adequately compensatefor the mode distortion. For a spatial mode distortion, themode-restoration device 24 can be a spatial light modulator (e.g., adevice sold as model SLM-100 by Santec Corporation). For a modedistortion caused by a group velocity dispersion in the atmosphere, themode-restoration device 24 may include a group-velocity dispersioncompensation fiber.

The pump pulses can be directly created in a single mode by using amode-locked laser whose output is typically transform-limited pulses. Incertain embodiments, the pump pulses are further filtered using aspectral filtering device to reduce the background noise, to increasethe pulse width, or to reshape the pulse profile. Another way to createsuch pump pulses in a single mode with desirable mode profiles is to usean optical arbitrary waveform generator (hereinafter “OAWG”), such asthe one disclosed in U.S. Application Publication No. US 20080089698 A1,entitled “Optical arbitrary waveform generation and processing usingspectral line-by-line pulse shaping”, the disclosure of which isincorporated herein in its entirety. An exemplary implementation of theOAWG involves the following steps. First, a narrowband laser incontinuous waves (such as those with spectral bandwidth less than 100kHz) is modulated by an optical comb generator (e.g., an optical combgenerator sold as model WTAS-02 by Optical Comb, Inc.) to create opticalfrequency comb lines around the central wavelength of the input laser.Second, the combs may be amplified and passed through a suitable opticalprocessor, such as the device known as “WaveShaper” (Model: 16000S,Manufacturer: Finisar), where the amplitude and phase of each comb lineis modulated individually and then multiplexed at a single output tocreate pump pulses in the desired phase and amplitude profiles. Third,the created phase and amplitude profiles may be measured using either orboth an optical spectrum analyzer and/or a frequency-resolving opticalgating device (e.g., FROG devices available from Coherent SolutionsInc.). The measurement results may then be compared with the desirablephase and amplitude profiles. If the results are not acceptable, thecomb lines' amplitude and phase may be adjusted in the optical processor(e.g., the WaveShaper device) to improve the resulting pulse profiles asmeasured by either or both the optical spectrum analyzer and/or thefrequency-resolving optical gating device. This procedure may beiterated until acceptable amplitude and phase profiles are measuredand/or attained.

In one embodiment, the use of the OAWG discussed above is applicableonly to pump pulses in the telecom C-band. To create optimized pumppulses in other wavelength bands, one can use frequency conversion innonlinear media, such as sum-frequency generation, difference-frequencygeneration, four-wave mixing bragg scattering, and four-wave mixingwavelength conversion, to convert the created telecom pulses into adesirable spectral band.

In one embodiment, the single-mode pump pulses are used to create twinbeams, signal and idler, in an optical parametric downconverter 14. Forthe downconverter 14, there is a wide range of commercial products tochoose from. For example, it can be a lithium niobate crystal orwaveguide (e.g., those manufactured by HC Photonics Corp. or ADvR Inc.),a potassium titanyl phosphate waveguide or crystal (e.g., thosemanufactured by Raicol Crystals Ltd.), a lithium triborate waveguide orcrystal (e.g., those manufactured by Raicol Crystals Ltd.), a bariumborate waveguide or crystal (e.g., those manufactured by Raicol CrystalsLtd.), an piece of optical fiber (e.g., those manufactured by NewportCorporation), a silicon nanowire on chip (such as the one used in XiangZhang, Bryn Bell, Mark Pelusi, Jiakun He, Wei Geng, Yunchuan Kong,Philipp Zhang, Chunle Xiong, and Benjamin J. Eggleton, “High repetitionrate correlated photon pair generation in integrated silicon nanowires,”Applied Optics Vol. 56, pp. 8420-8424 (2017)), or a hydrogenatedamorphous silicon waveguide on chip (such as the one used in Ke-YaoWang, Vesselin G. Velev, Kim Fook Lee, Abijith S. Kowligy, Prem Kumar,Mark A. Foster, Amy C. Foster, and Yu-Ping Huang, “Multichannelphoton-pair generation using hydrogenated amorphous silicon waveguides,”Opt. Lett. 39, 914 (2014)). For typical lithium niobate waveguides, thephase matching bandwidth is 20 nm or more for the downconversion intothe telecom spectral bands. For example, using a 775-nm pump laser,signal and idler photons can be created in pairs in a spectral rangebetween 1540 nm and 1560 nm. Thus, using pump pulses with 100 ps pulsewidth, signal and idler beams can be created, each containing about 100modes. For some highly-nonlinear optical fibers and hydrogenatedamorphous silicon waveguides, the phase matching bandwidth can be over50 nm. The use of longer pump pulses and/or nonlinear parametric deviceswith broader phase-matching bandwidth allows more signal and idler modesto be used, thus achieving a high detection SNR against broadbandbackground noise.

As illustrated in FIGS. 1 and 2, the signal beam is sent to interactwith the target, while the idler beam is retained using an opticalmemory or a delay line 16, 16 a. Depending on the need, the idler beamcan be amplified in an optical amplifier 16 b, such as an erbium-dopedfiber amplifier. The reflected signal beam and the idler beam aretemporally aligned and combined in a parametric upconverter 18. Intypical applications, the reflected signal beam is much weaker than theidler beam and is accompanied with significant background noise.

With reference to FIGS. 1 and 2, the parametric downconverter 14, whichgenerates the signal and idler beams, and the parametric upconverter 18,where the idler and returning signal beams are combined to createsignature photons, can each be implemented in nonlinear optical deviceshaving identical or similar nonlinear optical properties. Alternatively,the parametric downconverter 14 and the parametric upconverter 18 canalso be implemented using a signal nonlinear optical device, where thetwo processes can be temporally multiplexed or simultaneously occur butalong counter-propagation directions in the devices by using opticalcirculators.

FIG. 3 shows a design for the counter-propagation configuration realizedin a nonlinear waveguide 26 in accordance with one embodiment. The pumppulses enter the waveguide 26 from the left through an opticalcirculator 28. The signal and idler beams are created via parametricdownconversion as the pump propagates to the right, and exit to theright via another optical circulator 30. Meanwhile, the idler andreflected signal created from the previous pump pulses enter the samewaveguide 26 from the right via the second optical circulator 30. As theidler and reflected signal travel to the left, the photons in thereflected signal beam are up-converted to the signature photons, whichexit via the first circulator 28. This counter-propagation schemeensures that the upconversion process is phase coherent for the signaland idler beams once they are temporally aligned. In other embodiments,the foregoing scheme is implemented using separate nonlinear opticaldevices with similar nonlinear optical properties, such as nearlyidentical phase matching curves.

Referring back to FIGS. 1 and 2, the converted photons pass through asingle-mode filter 20 which selects the same or similar mode as the pumppulses driving the parametric downconverter 14. In one embodiment, thefilter 20 includes a band-pass spectral filter and a time shutter. Inother embodiments, the filter 20 may be implemented using a quantumfrequency conversion process.

Using the process illustrated in FIG. 1, the detection efficiency of thereflected signal photons is much higher than that for background photonswithout prior quantum correlation with the idler beam. To illustratethis, one can consider the following idealized parametric downconversionprocess: â→Σ_(j=1) ^(M){circumflex over (b)}_(j)ĉ_(j), where a pumpphoton in a single mode d creates a pair of signal and idler photonssimultaneously in M paired modes {{circumflex over (b)}_(j),ĉ_(j)} withequal probability. There are many photons in each pump pulse, so thatthere are on average many signal-idler photon pairs created at the sametime, with nearly equal population distribution across many modes. Inthis case, while the individual photons in each mode of either signal oridler beams do not have fixed phase values relative to photons in othermodes of the same beam, the paired photons in the two beams are in phasewith other photon pairs in the same or different modes created by thesame pump pulse. In other words, the total phase of the signal and idlerphotons in a pair is substantially uniform across many modes. This isbecause all photon pairs are created through a parametric nonlinearprocess driven by the same parent pump pulse. Because of this phaserelation, when recombined in the same nonlinear device or a separatedevice but with nearly identical phase matching properties, they willundergo the following nonlinear process: Σ_(j=1) ^(M){circumflex over(b)}_(j)ĉ_(j)→â, to create signature photons, which is the time reversalof the twin-beam generation process. The number of the created signaturephotons is then given by N_(c)˜|Σ_(j=1) ^(M){circumflex over(b)}_(j)ĉ_(j)|². This means if each mode has a mean-photon number n, thecreated signature photon number is proportional to N_(c)˜M²n.

In contrast, for the background photons, the upconversion is not phasecoherent, because they are not paired with the idler beam and haverandom phase. As a result, the number of the created signature photonsis given by the phase incoherent summary over the modes. If each modehas a background photon occupation of n, which yields the same averagephoton numbers with the above reflected signal, the created signaturephotons under the same detection condition will amount to N_(i)˜Mn. Itis clear that N_(i)=N_(c)/M, i.e., under the condition of the samemodes, the same photon occupation in each mode, and using the samedetection system, the detection efficiency for the reflected signalphotons is M times higher than that for background photons. This benefitcomes from the fact that the conversion process is phase coherent forthe returning signal beam, but not for the background photons.

In the examples illustrated in FIGS. 1 and 2, the signature photons arecreated through an upconversion process, where the one signal photon andone idler photon is combined to create a signature photon whose energyis the sum of the signal and idler photons. In alternate embodiments,other parametric conversion processes may be used to achieve the samefunctionality. For example, the signature photons can be created througha difference frequency generation or four-wave mixing process betweenthe signal and idler photons. In all possible realizations, thereturning signal photons in different modes need to phase coherentlyconverted to signature photons in a single or a few modes.

FIG. 4 shows another embodiment also based on nonlinear optics, but witha seedling idler beam. First, a pump pulse train 32 is created with eachpulse corresponding to a single time-frequency and spatial mode. Anidler beam 34 is created with many photons in a single spatial mode butroughly equally distributed among many time-frequency modes. An idlerbeam of this property can be generated via amplified spontaneousemission in an optical amplifier without any input laser. Depending onthe application, the amplifier output sometimes needs to pass additionalspectral filtering and/or temporally modulating devices to create theidler beam in desirable spectral and temporal modes. The idler beam 34is aligned and combined with the pump pulse train at a beam combiner 36,which can be a dichroic mirror, a wavelength-division multiplexer, or agrating filter. The combined beam is then guided into adifference-frequency generator 38, where a signal beam is createdthrough a difference-frequency generation (DFG) process. In oneembodiment, the difference frequency generator 37 is a nonlinearparametric device such as a lithium niobate crystal or waveguide (e.g.,those manufactured by HC Photonics Corp. or ADvR Inc.), a potassiumtitanyl phosphate waveguide or crystal (e.g., those manufactured byRaicol Crystals Ltd.), a barium borate waveguide or crystal (e.g., thosemanufactured by Raicol Crystals Ltd.), a piece of optical fiber (e.g.,those manufactured by Newport Corporation), a silicon nanowire on chip(such as the one used in Xiang Zhang, Bryn Bell, Mark Pelusi, Jiakun He,Wei Geng, Yunchuan Kong, Philipp Zhang, Chunle Xiong, and Benjamin J.Eggleton, “High repetition rate correlated photon pair generation inintegrated silicon nanowires,” Applied Optics 56, 8420 (2017)), or ahydrogenated amorphous silicon waveguide on chip (such as the one usedin Ke-Yao Wang, Vesselin G. Velev, Kim Fook Lee, Abijith S. Kowligy,Prem Kumar, Mark A. Foster, Amy C. Foster, and Yu-Ping Huang,“Multichannel photon-pair generation using hydrogenated amorphoussilicon waveguides,” Opt. Lett. 39, 914 (2014)). To produce a singlebeam with many modes, the seedling idler beam's spectral width needs tobe comparable with or larger than the phase matching bandwidth of theDFG process driven by the pump. For typical lithium niobate waveguides,the phase matching bandwidth is 20 nm in the telecom spectral band.Thus, if using transform-limited pump pulses with 100 ps pulse width andsub-GHz bandwidth, and seedling idler pulses of the same pulse width but10 nm spectral bandwidth, a signal beam will be created in about 100modes while the idler beam is amplified. For typical hydrogenatedamorphous silicon waveguides, the phase matching bandwidth can be over50 nm. Using longer pump pulses or phase matching with larger bandwidth,the signal beam will be created in more modes.

Still referring to FIG. 4, the signal beam is sent to interact with atarget, while the idler beam is retained using an optical memory 16 a oran optical delay line 16. Depending on the need, the idler beam can beadditionally amplified in an optical amplifier 16 b, such as anerbium-doped fiber amplifier. The reflected signal beam is temporallyaligned with and converted to a signature beam by the idler beam in asum-frequency generation (SFG) device 40. As in the embodiment discussedabove and shown in FIG. 1 and FIG. 2, the SFG can be the same nonlinearoptical device or a separate nonlinear optical device but with similarnonlinear optical properties, such as nearly identical phase matchingcurves. The photons in the signature beam will be filtered and detectedsuch that only the signature photons in the same mode as the pump forthe DFG process are detected.

In the scheme illustrated in FIG. 4, the signal beam is first createdvia DFG and then detected via SFG. An equivalent scheme can beimplemented with the signal created via SFG and detected via DFG. It hasthe same benefits coming from the phase coherent conversion.

In one embodiment, the signal emission and receiving components of thesystems shown in FIGS. 1, 2 and 4 are mounted on a laser-scanningapparatus, so that the launching and receiving angles of the signal beamcan be scanned. Meanwhile, the relative delay between the paired signaland idler pulses is also scanned. Based on the number of signaturephotons detected as the angles and delay are scanned, three-dimensionalsurface profiles of surrounding objects can be reconstructed. Comparedto direct lidar or radar measurement, the systems based on thephase-coherent photon conversion can achieve a much higher signal tonoise ratio, a better measurement resolution, and are robust to apply ina complex environment with strong background noise. Example applicationsinclude surveillance and surrounding data collection for autonomousautomobiles, autonomous drones, and robotic machines, for which accurate3D images of the surrounding environment must be obtained reliablydespite unpredictable background disturbances, including foreign lightillumination on the detector whose strength is much higher than thereflected signal.

In another embodiment, the technique of phase-coherent photon conversionis utilized for detection and imaging under the constraint of low peakpower for the probe. This includes non-destructive imaging oflight-sensitive biomedical samples and light-reactive chemical samples,where the high peak power of the probe beam damages or destroys thesamples. By spreading the photons in the signal beam over a large amountof modes, the signal peak power can be made very low and suitable forprobing light-sensitive or light-reactive materials.

In another embodiment, the technique of phase-coherent photon conversionis utilized for covert communications. Referring to FIG. 5, asingle-mode pump 42 creates low-power signal and idler twin beams in aparametric downconverter 14, with paired photons spreading over M pairsof signal and idler modes (where M is between 10 and 10,000 in oneembodiment). The signal beam is mixed with a broadband noise beam 44,which spatially, temporally, and spectrally overlaps with the signalbeam, but contains more photons that randomly distributed across manymodes. The optical-power ratio between the noise and signal beam can beas high as 0.1*M. The mixed beams are then sent to a distant site, wherethey are received, manipulated to encode information, and sent back. Thereturning signal beam is then aligned and combined with the idler beamin a parametric upconverter 18 to phase coherently create signaturephotons, which are filtered and detected in a single mode. The returningnoise beam is also converted by the idler beam, but the conversionefficiency is much lower, due to the absence of phase coherence. In theideal case, the conversion efficiency for the signal beam is M timesthat of the noise beam, so that even when the noise beam is 0.1*M timesmore powerful than the signal beam, approximately 90% of the convertedsignature photons are from the signal beam. In this way, the signal beamcan be effectively separated from the noise beam to extract the encodedinformation. By holding the idler beam confidentially, one can prevent athird-party from detecting the signal beam, as it is hidden well in theco-propagating noise beam that has much higher power. This is becausewithout the idler beam, no third-party can preferably detect themanipulated signal beam against the noise beam. Because the noise beamis stronger than the signal beam in terms of the optical power, littleto no useful information can be extracted by the third-party.

In another embodiment for covert communications, two parties, Party Aand Party B, share secret and identical keys. Referring to FIG. 6, PartyA uses a part of the keys to create an idler beam 46 in many modes basedon phase encoding across the modes. By mixing the idler beam with asingle-mode pump 48 in a difference frequency generator 50, a low-powersignal beam is created whose intensity is below that of ambient light insurrounding atmosphere from, for example, blackbody radiation, sunlight,and lighting devices. The attenuated signal is modulated to encodeinformation and then transmitted through the atmosphere to Party B.Meanwhile, Party B uses the secret keys to create a nearly identicalcopy of the idler beam 52, which is then combined with the receivedsignal beam in a sum-frequency generator 54 to phase-coherently createsignature photons for filtering and detection 56. During this process,the ambient light will also mix with the copied idler beam in thesum-frequency generator 54 to create signature photons, but with a muchlower efficiency as there is no phase coherence. By choosing the properamount of modes, the transmitted power level of the signal beam can bemade much lower than the ambient light, so that its presence is coveredand undetectable by any third-party, but the detected power level of thesignal is much higher than the ambient light based on the phase coherentsum-frequency generation.

In another embodiment, the disclosed technique is used for deep tissueimaging, where three-dimensional images of deep tissues can bereconstructed through timing the reflected signals, even in the presenceof strong loss and scattering. This is because the detection of thereflected signals is efficient only when they are aligned with the idlerbeam accurately in both space and time.

All examples and conditional language recited herein are intended forpedagogical purposes to aid the reader in understanding the principlesof the invention and the concepts contributed by the inventor tofurthering the art, and are to be construed as being without limitationto such specifically recited examples and conditions. Moreover, allstatements herein reciting principles, aspects, and embodiments of theinvention, as well as specific examples thereof, are intended toencompass both structural and functional equivalents thereof.Additionally, it is intended that such equivalents include bothcurrently known equivalents as well as equivalents developed in thefuture, i.e., any elements developed that perform the same function,regardless of structure.

It will be understood that the embodiments described herein are merelyexemplary and that a person skilled in the art may make many variationsand modifications without departing from the spirit and scope of theinvention. All such variations and modifications are intended to beincluded within the scope of the invention.

1. A method for discriminatively detecting electromagnetic waves,comprising the steps of: generating a signal beam and an idler beam in aplurality of paired modes; transmitting the signal beam; receiving atleast a portion of the transmitted signal beam; temporally aligning andcombining the idler beam, or a copy thereof, with the at least a portionof the transmitted signal beam to form combined signal and idler beams;generating signature photons from the combined signal and idler beams,wherein the signature photons are not quantum-entangled; and filteringand detecting the signature photons in at least one of the plurality ofpaired modes.
 2. The method of claim 1, further comprising the step ofstoring the idler beam in an optical delay line or in a memory device.3. The method of claim 1, wherein the at least a portion of thetransmitted signal beam is reflected from a target object.
 4. The methodof claim 3, further comprising the step of scanning the angle at whichthe signal beam is transmitted and the time delay of the idler beam fromthe signal beam to remotely obtain three-dimensional informationregarding the object.
 5. The method of claim 1, wherein said step ofgenerating a signal beam and an idler beam is performed using aparametric downconversion process to generate the signal beam; andwherein said step of generating signature photons includes step ofperforming a phase coherent nonlinear process by subjecting the combinedsignal and idler beams to a parametric upconversion process, whichcorresponds to the time reversal of the parametric downconversionprocess.
 6. The method of claim 1, wherein said step of generating asignal beam and an idler beam is performed using a difference-frequencygeneration process by using a seedling beam; and wherein said step ofgenerating signature photons includes the step of subjecting thecombined signal and idler beams to a sum-frequency generation process,which is the time reversal of the difference-frequency generationprocess.
 7. The method of claim 6, wherein said step of generating asignal beam is performed using a single-mode pump and a seedling idlerbeam to generate the signal beam through the difference-frequencygeneration process.
 8. The method of claim 1, wherein said step ofgenerating a signal beam and an idler beam is performed using asingle-mode pump.
 9. The method of claim 8, wherein said filtering anddetecting step includes the step of filtering the signature photons inorder to keep only those in a mode that is substantially identical withthat of the single-mode pump.
 10. The method of claim 1, furthercomprising the step of optionally compensating for mode distortion ofthe signal beam.
 11. The method of claim 1, further comprising the stepof optionally amplifying the idler beam prior to the performance of saidaligning and combining step.
 12. The method of claim 1, furthercomprising the step of including a noise beam in the signal beam priorto its transmission for conducting a covert communication.
 13. Apparatusfor discriminatively detecting electromagnetic waves, comprising: agenerator for generating a signal beam and an idler beam in a pluralityof paired modes, a transmitter for transmitting the signal beam; areceiver for receiving at least a portion of the transmitted signalbeam; an aligning device for temporally aligning and combining the idlerbeam, or a copy thereof, with the at least a portion of the transmittedsignal beam so as to generate signature photons from the combined signaland idler beams, wherein the signature photons are notquantum-entangled; and a detector, including a filter, for detecting thesignature photons in at least one of the plurality of paired modes. 14.The apparatus of claim 13, further comprising a storage device forstoring the idler beam.
 15. The apparatus of claim 14, wherein saidstorage device includes an optical delay line or a memory device. 16.The apparatus of claim 13, further comprising at least one scanner forscanning the angle at which the signal beam is transmitted and the timedelay of the idler beam from the signal beam to remotely obtainthree-dimensional information relating to an object from which thesignal beam is reflected and received by said receiver.
 17. Theapparatus of claim 13, wherein said generator includes a parametricdownconverter to generate the signal beam; and wherein said aligningdevice includes a parametric upconverter, which corresponds to the timereversal of said parametric downconverter, to generate the signaturephotons.
 18. The apparatus of claim 13, wherein said generator includesa difference-frequency generator to generate the signal beam; andwherein said aligning device includes a sum-frequency generator, whichis the time reversal of the difference-frequency generator, to generatethe signature photons.
 19. The apparatus of claim 13, wherein saidgenerator includes a single-mode pump to generate the signal beam.
 20. Acovert communication method, comprising the steps of: sharing identicalsecret keys between first and second communication nodes; at the firstnode, creating a broadband idler beam based on at least one of thesecret keys and combining the idler beam with a single-mode pump togenerate a signal beam, the signal beam having an intensity that islower than the surrounding ambient light; encoding information in thesignal beam by modulating its electromagnetic wave properties;transmitting the modulated signal beam to the second node; creating acopy of the idler beam at the second node based on at least one of thesecret keys and using the copy of the idler beam to create signaturephotons from the signal beam received from the first node through aprocess corresponding to the time reversal of the signal beam generationprocess at the second node; and filtering and detecting the signaturephotons in a single mode resembling the pump.